Portable computing devices such as laptop computers, tablet PC's, email accessing devices, PDA's, etc. are becoming increasing popular and have become indispensable tools for many users. With the advancements in computer and network technologies, the portable devices are more and more powerful and versatile, and a great emphasis in computer hardware development is placed on further enhancing the mobility of such devices. Nevertheless, one major technical obstacle that has been hindering the mobility of portable devices is that those devices need power to operate, and replenishing the power typically requires the use of a power cord. For instance, a typical laptop computer is recharged by using a dedicated external power supply that functions as an AC-DC converter. Such a power supply is bulky and heavy, and may be unusable when the user travels to a region with a different AC voltage standard. Similarly, PDA's and email access devices often have to be recharged by placing them in a docking station. A frustrating scenario frequently experienced by users of portable devices is that the portable device runs out of power and its external power supply or charger is unavailable or unusable, rendering the portable device useless.
One solution that has been proposed to allow a portable device to be powered or recharged without the attachment of a power cord is to transfer power via inductive coupling. To that end, the portable device is provided with a secondary coil connected to its power supply circuit, and a powering device is provided with a surface that has at least one primary coil built into it. To power the device, the device is placed on the surface of the powering device such that the primary and secondary coils are in close proximity so that they can inductively interact with each other to form a transformer. The primary coil is energized at a selected frequency, such as 100 KHz, and power is transferred to the power supply circuit of the portable device via the inductive coupling between the primary and secondary coils.
One such inductive powering device for portable devices has been proposed in “Power Transmission of a Desk with a Cord-Free Power Supply,” Hoichi Hatanaka, et al., pp. 3329-3331, IEEE Transaction on Magnetics, vol. 38, no. 5, September 2002. In that example, a plurality of primary coils are distributed over the surface of a desk-type power station. Each primary coil has its own driving circuit sensing and powering circuit. The portable device has an active secondary coil with a 500 KHz oscillator. When the portable device is placed on the desk surface, the active oscillator of the secondary coil is activated so that the primary coils can sense the presence of the secondary coil. Only those primary coils overlapping with the secondary coil are then energized to transfer power to the portable device. One fundamental problem with this arrangement is that the oscillator circuit of the secondary coil requires power to operate. If the battery of the portable device is completely drained, there is no power available to operate the oscillator circuit. As a result, the portable device cannot be recharged, since the desk-type power station cannot sense its presence and thus will not energize the primary coils to transfer power to it. Another problem with this arrangement is that the number of drive circuits for the primary coils is proportional to area of the desk surface, and the cost for the drive circuits can be quite high for a large desk surface.